Why does gcc generate verbose assembly code?
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Jared Nash
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Published on 2012-09-08T09:34:36Z
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2012/09/08
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I have a question about assembly code generated by GCC (-S option). Since, I am new to assembly language and know very little about it, the question will be very primitive. Still, I hope somebody will answer:
Suppose, I have this C code:
main(){
int x = 15;
int y = 6;
int z = x - y;
return 0;
}
If we look at the assembly code (especially the part corresponding to int z = x - y ), we see:
main:
...
subl $16, %esp
movl $15, -4(%ebp)
movl $6, -8(%ebp)
movl -8(%ebp), %eax
movl -4(%ebp), %edx
movl %edx, %ecx
subl %eax, %ecx
movl %ecx, %eax
movl %eax, -12(%ebp)
...
Why doesn't GCC generate something like this, which is less copying things around.
main:
...
movl $15, -4(%ebp)
movl $6, -8(%ebp)
movl -8(%ebp), %edx
movl -4(%ebp), %eax
subl %edx, %eax
movl %eax, -12(%ebp)
...
P.S.
Linux zion-5 2.6.32-21-generic #32-Ubuntu SMP Fri Apr 16 08:10:02 UTC 2010 i686 GNU/Linux gcc version 4.4.3 (Ubuntu 4.4.3-4ubuntu5)
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